This invention relates generally to an electrostatographic printing machine and more particularly concerns an apparatus for controlling the quantity of release material metered to a heated fuser member.
In a typical electrostatographic printing machine, a latent image is recorded on a surface and developed with charged particles. The particles are transferred from the image bearing surface to a sheet of support material. Thereafter, the particles are permanently affixed to the sheet of support material forming a copy of the original document. Electrostatographic printing includes electrophotographic printing and electrographic printing. Both of the foregoing processes are similar to one another. Electrophotographic printing employs a photoconductive member which is charged to a substantially uniform level. A light image of the original document irradiates the charged photoconductive member dissipating the charge in the irradiated areas recording an electrostatic latent image thereon. Electrographic printing differs from electrophotographic printing in that a photoconductive member or a light image is not required to create a latent image of the original document. Generally, both of the foregoing processes employ heat settable particles which are permanently affixed to the sheet of support material by the application of heat thereto.
In order to permanently affix the particles to the sheet of support material, it is necessary to elevate or heat the particles to a point at which the constituents thereof coalesce and become tacky. This action causes the particles to be absorbed to some degree into the fibers of the sheet of support material which may be amongst others, plain paper or a sheet of thermoplastic material. After the particles cool, solidification of the particles produces a firm bond between the sheet of support material and the particles. The use of heat to permanently affix powder images onto a sheet of support material is well known in the art.
One approach for applying heat thereto is to pass a sheet of support material with the powder image thereon through a pair of opposed rollers at least one of which is heated. One type of system employing a pair of rollers utilizes a heated fuser roller having an outer surface covered with polytetrafluoroethylene, commonly known as Teflon, to which a release agent, such as silicone oil is applied. Preferably, the Teflon layer has a thickness of about several mils with the thickness of the oil being less than one micron. Silicone based oils, which possess a relatively low surface energy, have been found to be useful for heated fuser rolls having an outer surface of Teflon. Generally, a thin layer of silicone oil is applied to the surface of the heated roller to form an interface between the roll surface and powder image carried on the support material. The low surface energy of this layer prevents the particles from transferring to the roller. This insures that the particles remain on the sheet of support material so as to be permanently affixed thereto during the heating process.
Another approach has been to employ a metal heated roller which has a low molecular weight polyethylene applied thereto as a release agent. This release material is generally a solid at room temperature. Hereinbefore, it has been difficult to control or regulate the quantity of release material applied to the fuser roll. One approach taken is described in co-pending application Ser. No. 482,675 filed in 1974. As described therein, a composite doctor blade structure engages the fuser roll to meter a prescribed thickness of release material thereto. The doctor blade assembly is loosely supported enabling it to expand freely under temperature.
It is a primary object of the present invention to improve the structure employed to control the metering of release material to a heated fuser member.